1. Field of the Invention
The present invention relates to a transformer inrush current suppression apparatus for suppressing a transformer inrush current which occurs when a circuit breaker is closed.
2. Description of the Related Art
When a transformer is energized without any load by being connected to a power supply while there is residual magnetic flux in a transformer core, a large transformer inrush current is known to flow. The magnitude of this transformer inrush current is several times larger than the rated load current of the transformer. When such a large transformer inrush current flows, the system voltage fluctuates, and when this voltage fluctuation is large, this many affect electric power consumers.
Accordingly, there is known a method for suppressing a transformer inrush current, and this method uses a resistor-attached circuit breaker including a closing resistor and a contact which are connected in series. In this method, the resistor-attached circuit breaker is connected in parallel with a main contact of a circuit breaker. The resistor-attached circuit breaker is closed in advance of closing the main contact of the circuit breaker. Therefore, this suppresses the transformer inrush current (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-075145).
There is known another suppression method. In this method, when a directly-grounded three-phase transformer is energized with three single-phase circuit breakers, any one single-phase circuit breaker is closed in advance, and thereafter the remaining two single-phase circuit breakers are closed, to suppress the transformer inrush current (for example, John H. Brunke et al., “Elimination of Transformer Inrush Currents by Controlled Switching—Part I: Theoretical Considerations”, IEEE TRANSACTIONS ON POWER DELIVERY, IEEE, April, 2001, Vol 0.16, No. 2, p. 276-280).
There is also disclosed a method for suppressing a transformer inrush current when a non-effectively grounded three-phase transformer is energized with a three-phase collective-operation circuit breaker. In this method, a value of magnetic flux remaining in a core is measured when the transformer is disconnected. When the transformer is energized, the transformer inrush current is suppressed by controlling a closing phase of the circuit breaker (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2008-160100).
On the other hand, a Scott connection is known as a method for converting a three-phase alternating current voltage into single-phase alternating current voltages. In the Scott connection, two single-phase transformers are connected, so as to convert a three-phase alternating current voltage into two pairs of single-phase alternating current voltages. For example, a Scott connection transformer is used when electric power is supplied to a single-phase electric furnace or a single-phase alternating-current electric train.
However, the above methods for suppressing transformer inrush currents have the following problems.
In the method for suppressing a transformer inrush current using a resistor-attached circuit breaker, there is a problem in that it is necessary to add a resistor-attached circuit breaker to an ordinary circuit breaker. Therefore, the overall size of the circuit breaker is larger.
In any of the methods for suppressing the transformer inrush currents, it is not expected to disconnect the Scott connection transformer or the like that converts a three-phase alternating current voltage into two pairs of single-phase alternating current voltages.
For example, in the method for measuring the residual magnetic flux and controlling the closing phase of the circuit breaker, the method for controlling the three-phase transformer used for a power system cannot be directly applied to the Scott connection transformer. In the case of the Scott connection transformer, the magnetic flux of the transformer core cannot be directly calculated by measuring phase voltages or line-to-line voltages (voltages between lines) at the primary side.